Over fifty years ago it was recognized that alkylbenzene sulfonates (ABS) were quite effective detergents superior to natural soaps in many respects. Because of their lower price, their price stability, and their effectiveness in a wide range of detergent formulations, ABS rapidly displaced soaps in household laundry and dishwashing applications and became the standard surfactants for the detergent industry.
The alkylbenzene sulfonates had substantial branching in the alkyl chain until the early 1960's when it became apparent that these detergents were contributing to the pollution of lakes and streams and forming relatively stable foams. Examination of the problem showed that alkyl chains with a branched structure were not susceptible to rapid biodegradation and the surfactant properties of the detergent thus persisted for long periods of time. This was contrary to the earlier situation when natural soaps were used because the linear alkyl chains in natural soaps underwent rapid biodegradation.
After recognizing the biodegradability of ABS based on alkylation by linear olefins, industry turned its attention to the production of these unbranched olefins and their subsequent use in the production of linear alkylbenzenes. Processes were developed for efficient alkylation of benzene by available feedstocks containing linear olefins, and the production of linear alkylbenzenes (LABs) became another reliable process broadly available to the petroleum and petrochemical industry. It gradually evolved that HF-catalyzed alkylation was particularly effective in LAB production, and an HF-based alkylation process became the industry standard.
At this point the definition of several terms are necessary to adequately understand and appreciate what follows. Alkylation typically is performed using an excess of benzene relative to olefins. The ideal process would afford 100% conversion of olefins using an equimolar proportion of benzene and olefins, but since this is not attained one strives for maximum olefin conversion using a benzene to olefin molar ratio up to about 30. The better the process, the lower will be the benzene:olefin ratio at a high conversion of, say, 98%. The degree of conversion at a constant value of benzene-olefin ratio is a measure of catalytic activity (subject to the caveat that the ratio must not be so high that the degree of conversion is invariant to small changes in this ratio). The degree of conversion may be expressed by the formula, ##EQU1## where V equals percent conversion, C equals moles of olefin consumed, and T equals moles olefin initially present.
However active the catalyst may be, a process based on the catalyst also must be selective. Selectivity is defined as the percentage of total olefin consumed under reaction conditions which appears as monoalkylbenzene and can be expressed by the equation, ##EQU2## where S equals selectivity, M equals moles of monoalkylbenzenes produced, and C equals moles olefin consumed. The better the selectivity, the more desirable the process. An approximate measure of selectivity is given by the equation, ##EQU3## where "total products" includes monoalkylbenzenes, polyalkylbenzenes, and olefin oligomers. At high selectivity (S&gt;85%) the results calculated from the two equations are nearly identical. The latter of the foregoing two equations is routinely used in commercial practice because of the analytical difficulty in distinguishing between oligomers and polyalkylbenzenes.
Finally, the reaction of linear olefins with benzene in principal proceeds according to the equation, EQU C.sub.6 H.sub.6 +R.sub.1 CH=CHR.sub.2 .fwdarw.C.sub.6 H.sub.5 CH(R.sub.1)CH.sub.2 R.sub.2 +C.sub.6 H.sub.5 CH(R.sub.2)CH.sub.2 R.sub.1.
Note that the side chain is branched solely at the benzylic carbon and contains only one branch in the chain. Although strictly speaking this is not a linear alkylbenzene, nonetheless the terminology which has arisen for the process and product in fact includes as linear alkylbenzenes those materials whose alkyl group chemically arises directly from linear olefins and therefore includes alpha-branched olefins. Because alkylation catalysts also may induce the rearrangement of olefins to ultimately give products which are not readily biodegradable (vide supra), for example, .alpha.,.alpha.-disubstituted olefins which subsequently react with benzene to afford an alkylbenzene with branching at other than the benzylic carbon, ##STR1## the degree to which the catalyst effects formation of linear alkylbenzenes is another important catalyst parameter. The degree of linearity can be expressed by the equation, ##EQU4## where D equals degree of linearity, L equals moles of linear monoalkylbenzene produced, and M equals moles of total monoalkylbenzene produced.
Consequently, the ideal process is one where V equals 100, S equals 100, and D equals 100. The linearity requirement is assuming added importance and significance in view of the expectation in some areas of minimum standards for linearity in detergents of 92-95% near-term, increasing to 95-98% by about the year 2000. Since the olefinic feedstock used for alkylation generally contains a small percentage of non-linear olefins--a non-linear olefin content of about 2% is common to many processes--the requisite linearity in the detergent alkylate places even more stringent requirements on catalytic performance; the inherent linearity of the alkylation process must increase by the amount of non-linear olefins present in the feedstock. For example, with a feedstock containing 2% non-linear olefins the catalyst must effect alkylation with 92% linearity in order to afford a product with 90% linearity, and with a feedstock containing 4% non-linear olefins the catalyst must effect alkylation with 94% linearity to achieve the same result.
The invention described and claimed within leads, inter alia, to increased linearity of alkylbenzene sulfonates. Although the result of increased linearity is simple, clear, and incontrovertible, its origin is at first blush obscure and mysterious, and we need to take a brief sojourn into the origin of the linear olefins used as the feedstock for LAB processes so that we may see in bold relief the problem which we faced and appreciate how its solution, which is our invention, leads to a substantial improvement in LAB processes.
The linear olefins used to react with benzene in a LAB process generally arise from the dehydrogenation of linear paraffins, or normal paraffins. Generally the dehydrogenation reaction is not run to completion in order to minimize cracking, isomerization, and other byproducts, and the entire dehydrogenation product mixture is used as the feedstock to an alkylation zone. The polyolefins formed during dehydrogenation are minimized when the dehydrogenation product mixture is used as the alkylation feedstock for LAB production, often by a separate selective hydrogenation process. Consequently the alkylation feedstock is a mixture largely of unreacted paraffins, small amounts (approximately 2% or less) of branched olefins, unbranched and linear monoolefins of the same carbon number (typically C6-C20 range) as the normal paraffins which are dehydrogenated, and small amounts of aromatic byproducts which have the same carbon number as the paraffins which are dehydrogenated, that is, C6-C20 aromatics. Although it has been known for some time that these aromatic byproducts are formed in the catalytic dehydrogenation of paraffins (e.g., see the article starting at page 86 of the Jan. 26, 1970 issue of "Chemical Engineering" ), we have only recently observed that they have a significant deleterious effect on a LAB process. Therein hangs our tale, and we now turn our attention to these previously recognized but unappreciated aromatic byproducts of normal paraffin dehydrogenation.
Although our invention is generally applicable to alkylation using linear olefins in the C6-C20 range, for LAB production alkylation feedstocks containing C8-C16 olefins, and especially C10-C14 olefins, are most preferred, and for simplicity of presentation and clarity of exposition we shall focus on the C10-C14 materials with the proviso that our description is applicable, with only minor modifications if any, to the broader range of materials of C6-C20. The aromatic byproducts in question include alkylated benzenes, polyalkylbenzenes, naphthalenes, other polynuclear aromatic hydrocarbons, alkylated polynuclear hydrocarbons in the C10-C14 range, indanes, and tetralins, that is, they are aromatics of the same carbon number as the paraffin being dehydrogenated and may be viewed as aromatized normal paraffins. In the general case the aromatic byproducts contain from 6 up to about 20 carbon atoms, but because the C6 member is benzene, which is the aromatic most often being alkylated, it is only the aromatic byproducts of 7 through about 20 carbon atoms which concern us here. Typically these aromatic byproducts are formed to the extent of perhaps 0.2-0.7% in a dehydrogention unit. However, as the flow scheme typical for LAB processes of FIG. 1 shows, at least a portion of the unreacted alkylation feedstock is recycled to the dehydrogenation unit, leading to the accumulation of the aromatic byproducts so that at steady state conditions they are present in the alkylation feedstock at concentrations typically on the order of 3-6 weight percent where HF is the alkylation catalyst and 4-10 percent where a solid alkylation catalyst is used.
We have observed that the aromatic byproducts, especially at their steady state concentration, in the alkylation feedstock substantially reduce the activity of an alkylation catalyst, thereby substantially reducing the useful lifetime (stability) of an alkylation catalyst. A common measure of catalyst stability in detergent alkylation is the number of hours the catalyst will afford 100% conversion at an otherwise unvarying set of reaction conditions (benzene to olefin ratio, temperature, space velocity, etc.). With increasing concentration of aromatic byproducts in the alkylation feedstock the number of hours a catalyst effects 100% conversion decreases. Conversely, reducing the concentration of the aromatic byproducts in the alkylation feedstock increases catalyst stability, i.e., the number of hours a catalyst effects 100% conversion increases.
The number of hours a particular catalyst will continue to effect 100% conversion increases with temperature. Therefore, the customary solution to a decrease in catalyst stability is to increase operating temperature. The corollary of this is that if an increase in catalyst stability can be effected by an independent means then the operating temperature may be decreased. Therefore, a consequence of the resulting increased catalyst stability arising from a reduced concentration of aromatic byproducts in the alkylation feedstock is that one can decrease alkylation temperature without any adverse effects. Recently we have observed that the degree of linearity in linear alkylbenzenes is more highly dependent on the alkylation temperature than on, for example, the nature of the alkylation catalyst. Therefore, any process change which permits a lower alkylation temperature leads to an increase in linearity of LABs. It follows that a consequence of reducing the concentration of aromatic byproducts in the alkylation feedstock is to afford LABs with a higher linearity because of a reduction in alkylation temperature made possible by the increase in catalyst stability attending the reduced aromatic byproduct concentration.
Thus our invention is simple; reduce the concentration of aromatic byproducts present in alkylation feedstocks used for LAB production. The result of our invention is simply and significantly an increased linearity of LABs. These favorable results arrive in a highly circuitous fashion which the foregoing explanation hopefully makes clear and unmistakable.
As important as is the increased linearity afforded by our invention it is not the sole benefit conferred, for another outcome of reducing aromatic byproducts is an increased selectivity of alkylation. Since the olefins in the alkylation feedstock constitute a high value item the increased selectivity translates directly to an economic benefit. As will also become apparent from the following description there are still other benefits ancillary to our invention which makes its practice highly advantageous.